The present invention relates to a method of polarimetry for use in an identification, examination on the purity, determination of the concentration, and the like of a solute in a sample solution.
A polarimeter is employed as an optical rotation detecting type saccharimeter for detecting the concentrations of fructose, sucrose, glucose, and the like contained in an aqueous solution. Such a polarimeter can also determine especially the concentrations of spontaneously optical active substances such as glucose and protein in a urine and, therefore, is expected to come into wide use as a urinalysis equipment which requires no consumable items such as test papers.
FIG. 7 shows a conceptual constitution of one example of conventional polarimeters. The polarimeter is for determining the magnitude of spontaneously optical rotatory power, i.e., an angle of rotation attributed to a spontaneously optical rotatory power of a spontaneously optical active substance in a sample. In concrete, the angle of spontaneously optical rotation is determined on the basis of an angle of magneto-rotation (so called compensated value) by an optical Faraday effect when the spontaneously optical rotation attributed to the spontaneously optical active substance is canceled (compensated) by the magneto-rotation.
In the polarimeter, a light source 14 for projecting a substantially parallel light, configured with a sodium lamp, a band-pass filter, a lens, a slit and the like, projects a substantially parallel light composed of, for example, a sodium D ray having a wavelength of 589 nm. A polarizer 15 transmits only a component that has a specific plane of vibration out of the incident light projected from the light source 14.
A sample cell 16 for holding a sample has a pair of mutually opposing transparent transmission surfaces, and is arranged so that the light projected from the light source 14 can transmit through the inside thereof. An analyzer 17 transmits only a component that has another specific plane of vibration out of the light transmitted through the sample cell 16. The relative angle Θ formed between the transmission axis of the polarizer 15 and the transmission axis of the analyzer 17 is fixed at π/2.
A photosensor 18 detects the component transmitted through the analyzer 17 out of the light projected from the light source 14. A Faraday cell 19 functions as an optical modulator for modulating and controlling the plane of vibration of the light projected from the light source 14 on the basis of a modulation signal outputted from a signal generator 23 and a control signal outputted from a computer 22. The Faraday cell 19 is driven by a Faraday cell driver 20.
Further, a lock-in amplifier 21 performs a phase sensitive detection on the output signal from the photosensor 18 by using the modulation signal outputted from the signal generator 23 as a reference signal. The computer 22 calculates the angle of rotation attributed to the sample accommodated in the sample cell 16 on the basis of the control signal, and the output signal from the lock-in amplifier 21.
As described above, by sweeping the angle of the plane of vibration by the Faraday cell, it becomes possible to achieve simplification and compactness thereof as compared with apparatuses using other means for modulating the plane of vibration.
The principle of the conventional polarimeter will be described in the followings.
In first, the polarization direction is modulated with an amplitude=“δ” and an angular frequency of “ω” in the Faraday cell 19. In this step, the intensity “I” of the light that reaches the photosensor 18 is represented by the following equation (6):I=T×I0×(COS(Θ−α+β+δ×SIN(ω×t)))2  (6)where “T” denotes a transmittance of the sample, “I0” denotes an intensity of the light incident upon the sample, “Θ” denotes a relative angle formed between the optical axes of the polarizer 15 and the analyzer 17, “α” denotes an angle of rotation attributed to the sample, “β” denotes an angle of rotation due to the Faraday cell 19, and “t” denotes the time). It is noted that their respective transmission and reference losses of the sample cell and the analyzer are ignored.
In the equation (6), Θ is fixed to be π/2, and hence the following equation (7) is given:I=T×I0×(SIN(β−α+δ×SIN(ω×t)))2  (7)
Herein, in case of β−α=0, in other words, when the angle of rotation of the polarization direction due to the optical rotation is compensated by the angle of rotation due to the Faraday cell 19, the equation (7) is expressed as the following equation (8):I=(½)×T×I0×(1−cos(2×δ×SIN(ω×t)))(½)×T×I0×(1−(J0(2×δ)+2×J2(2×δ)×COS(2×ω×t)+ . . . ))  (8)where Jn(x) is an nth-degree Bessel function).
The equation (8) indicates that “I” does not contain the modulation frequency component “ω” in this case. Approximately considering this, i.e., assuming that the angle of rotation attributed to the sample and the amplitude of the modulation are small, |β−α|<<1, and δ<<1, the equation (7) is approximated to the following equation (9).                                                         I              ≈                            ⁢                              T                ×                                  I                  0                                ×                                                      (                                          β                      -                      α                      +                                              δ                        ×                                                  SIN                          ⁡                                                      (                                                          ω                              ×                              t                                                        )                                                                                                                )                                    2                                                                                                        =                            ⁢                              T                ×                                  I                  0                                ×                                  (                                                                                    (                                                  β                          -                          α                                                )                                            2                                        +                                          2                      ×                                              (                                                  β                          -                          α                                                )                                            ×                      δ                      ×                                              SIN                        ⁡                                                  (                                                      ω                            ×                            t                                                    )                                                                                      +                                                                                                                                        ⁢                                                                    (                                          δ                      ×                                              SIN                        ⁡                                                  (                                                      ω                            ×                            t                                                    )                                                                                      )                                    2                                )                                                                                        =                            ⁢                              T                ×                                  I                  0                                ×                                  (                                                                                    (                                                  β                          -                          α                                                )                                            2                                        +                                          2                      ×                                              (                                                  β                          -                          α                                                )                                            ×                      δ                      ×                                              SIN                        ⁡                                                  (                                                      ω                            ×                            t                                                    )                                                                                      +                                                                                                                                        ⁢                              (                                                                            δ                      2                                        /                    2                                    ×                                      (                                          1                      -                                              COS                        ⁡                                                  (                                                      2                            ×                            ω                            ×                            t                                                    )                                                                                      )                                                  )                                                                        (        9        )            
This indicates that respective signal components with angular frequencies of 0 (DC), “ω” and “2×ω” are present in the output signal “I” from the photosensor. By the phase sensitive detection of the signal “I” using the modulation signal as a reference signal in the lock-in amplifier, it is possible to pick up the component of the angular frequency “ω”, i.e., the signal “S” shown by the following equation (10):S=T×I0×2×(β−α)×δ  (10)
This signal “S” equals to zero only in case of β=α, and the signal “S” equal to zero denotes the extinction point. A polarized light is rotated, in other words, “β” is controlled by the Faraday cell 19 to obtain “β” when “S” becomes zero. The resulting value of “β” is the angle “α” of rotation. The same is also true for the case where this process is considered on the basis of the equation (8). The output signal becomes zero upon the phase sensitive detection of the value “I” in case of β=α. Therefore, “β” is controlled so that “S” becomes zero. Then, the angle “α” of rotation is determined from the value of “β” at this step.
As described above, by modulating the angle of plane of vibration of light, it is possible to pick up only the signal “S” of the modulated frequency component selectively while separating the signal from noises attributed to an intensity of the light source, a fluctuation and radiation of the power source, and the like, thereby deriving a signal with a high S/N ratio. Therefore, the extinction point can be determined accurately by using this value of the signal “S”, and hence the angle “α” of rotation can be determined with high precision. Simultaneously, control of the polarization direction eliminates the necessity of a large scale mechanical mechanism.
On the other hand, as a conventional examination method for examining glucose, protein and the like in a urine, there is a method in which a test paper or the like containing a reagent is dipped in a urine, and the color reaction thereof is observed by means of a spectroscope or the like. This method requires the use of a consumable article such as a test paper. However, if the angle of rotation attributed to the urine is measured by means of the high precision polarimeter described above, it is possible to detect the angles of rotation attributed to optical active substances present in the urine at a low concentration such as glucose and protein. Consequently, it is possible to calculate the concentrations thereof on the basis of the detected values. As a result of this, it becomes possible to examine the glucose and protein concentrations in a urine without any consumable article.
However, in the above-described method, if there occur microparticles such as bubbles and dust in the optical path for the substantially parallel light in the sample cell when “β” is controlled so that “S” becomes zero, the feedback loop is not stabilized. Accordingly, additional time may be taken to obtain the measurement result, and hence the duration of the measurement time is not stabilized.
Further, a feedback loop is preferably constructed to control “β” so that “S” becomes zero. For this reason, “β” is desirably changed continuously.
In view of the foregoing problems in the prior art, it is therefore an object of the present invention to provide a method of polarimetry which is less susceptible to the influences of microparticles such as bubbles and dust, provides a constant measurement time, and has high reliability.